H-bridge electronic phase converter

ABSTRACT

A phase converter that converts single phase AC electric power to balanced three phase AC power. Two input terminals are connectable to a single phase AC power source, and connect directly to two output terminals of the converter. The phase converter has a storage capacitor, a charging circuit for controlled charging the storage capacitor and an output circuit for controlled discharge of the storage capacitor to provide single phase AC power to a third output terminal. The charging circuit controls input to the storage capacitor to provide a sinusoidal input current and to step up the voltage to the storage capacitor. The output circuit provides output power to the third output terminal of a predetermined phase and amplitude, relative to the other two output terminals, to result in balanced three phase AC power at the three output terminals.

This application claims the benefit under 35 U.S.C. §119(e) of the U.S.provisional patent application No. 60/334,418 filed Nov. 28, 2001.

TECHNICAL FIELD

The present invention relates to phase converters and more particularlyto a phase converter for that uses one or more H bridges for convertingsingle phase AC power to three phase AC power.

BACKGROUND ART

Three phase AC motors are generally simpler, more reliable and moreefficient than single phase AC motors. In addition to three phase ACmotors, much high-power industrial equipment requires three phase ACpower. Three phase AC power is generally supplied to industrial areas.However, only single phase AC power is available to most residential andrural areas.

The single phase AC power available in most residential and rural areasis provided by a step down transformer connected a high voltage lineand, in the United States, is normally supplied as about 240 volts at 60Hz between the first and second input lines. The transformer isgenerally center tapped with a neutral line to provide two phases ofabout 120 volts that are separated by 180 degrees.

Phase converters and inverters convert single phase AC power to threephase AC power to power three phase motors. Phase converters generate asecond voltage that is out of phase with the input voltage. The firstphase is the voltage between the first and second input line, the secondphase is the voltage between the first input line and the second voltageand the third phase is the voltage between the second input line and thesecond voltage. Three equal phases spaced 120 degrees apart are providedif the second voltage has an amplitude of {square root over (3)}/2 timesthe amplitude of the input voltage and is 90 degrees out of phase withthe input voltage.

The two types of phase converters generally available are the staticphase converter and the rotary phase converter. In prior known staticphase converters for use with inductive loads two terminals from theinput supply were connected to two of the windings of a three phasemotor and a capacitor was connected in series between the third windingand one of the terminals from the input supply. The capacitor incombination with the inductive load creates a lead circuit to providethe out of phase second voltage.

Such phase converters are disclosed in U.S. Pat. No. 4,492,911 toMolitor, U.S. Pat. No. 4,777,421 to West, U.S. Pat. No. 3,673,480 toJohnstone and U.S. Pat. No. 5,621,296 to Werner et al. This type ofphase converter includes a large capacitor for starting the motor and asmaller capacitor for running the motor. This type of phase converter isrelatively inexpensive, however this type of phase converter can only beused with inductive loads. The capacitor must be selected for thespecific inductive load to provide the correct phase shift. Also, theamplitude of the voltage out of the capacitor is at most one half theinput voltage so this type of phase converter cannot provide balancedcurrents to the windings at varying loads. Unbalanced currents causelocalized heating so that three phase motors run with this type ofstatic phase converter can only be run at a fraction of the ratedcapacity.

U.S. Pat. No. 5,293,108 to Spudich discloses a static phase converterthat includes a balancing coil between the two input lines and acapacitor connected between one input line and the third winding toshift the phase. As in the previously described static phase converters,two terminals from the input supply were connected to the first andsecond windings of a three phase motor, and a start capacitor and asmaller run capacitor are provided. The balancing coil and capacitormust be selected to match the impedance of the three phase load withthis converter.

Rotary phase converters use motor-generators powered by single phase ACpower to generate the second voltage signal. Rotary phase converters aredisclosed in U.S. Pat. No. 4,656,575 to West, U.S. Pat. No. 5,065,305 toRich, and U.S. Pat. No. 5,187,654 to Felippe. Rotary phase convertersare generally more complex, more expensive and less efficient thanstatic phase converters, and produce an unbalanced output which causessevere imbalances in the phase currents of three phase motors.

Inverters convert the entire single phase AC input voltage to a DCvoltage with rectifiers and convert the DC voltage into three balancedAC phases with an inverter circuit. Examples of inverters are disclosedin U.S. Pat. No. 4,855,652 to Yamashita et al., U.S. Pat. No. 5,793,623to Kawashima et al., U.S. Pat. No. 4,849,950 to Sugiura et al. and U.S.Pat. No. 4,978,894 to Takahara. The inverter circuit requires a minimumof six transistors and six diodes as well as control electronics for allof the transistors. Inverters are generally more complex and moreexpensive than static phase converters. Since the entire single phase ACinput voltage is converted to DC, inverters are inherently lessefficient than static phase converters. The output voltage of invertersconsists of a pulse-width-modulated (PWM) signal with a high harmoniccontent, limiting their application to inductive loads. The highfrequency harmonics present in the output voltages cause unwantedreflections in the wires connecting the inverter to the motor load, andlimit the acceptable distance between the inverter and the motor.

U.S. Pat. No. 6,297,971 to Meiners, incorporated herein by reference,discloses a phase converter that draws a sinusoidal current from thepower source and provides sinewave voltages to all terminals.

DISCLOSURE OF THE INVENTION

A phase converter for converting single phase AC power to balanced threephase power AC includes a charging circuit, a storage capacitor, anoutput circuit and a controller. The charging circuit is connected to anAC power source and includes means for rectifying and stepping up theinput voltage to charge the storage capacitor. The charging circuitincludes switches that are switched by control electronics at arelatively high frequency with a selected variable duty cycle to providea sinusoidal input current from the AC power source. The output circuitincludes first, second and third output terminals and means, connectedto the storage capacitor and to the third output terminal, for providinga selected AC output power signal to the third output terminal from thestorage capacitor. The first output terminal connects to the first inputterminal from the AC power source and the second output terminalconnects to the second input terminal from the AC power source.

BRIEF DESCRIPTION OF THE DRAWINGS

Details of this invention are described in connection with theaccompanying drawings that bear similar reference numerals in which:

FIG. 1 is a schematic circuit diagram of a phase converter embodying thefeatures of the present invention.

FIG. 2 is a detailed schematic circuit diagram of the controller of FIG.1.

FIG. 3 is a schematic circuit diagram of an another phase converterembodying the features of the present invention.

FIG. 4 is a detailed schematic circuit diagram of the controller of FIG.3.

FIG. 5 is a graphical representation of the output voltages of the phaseconverter of the present invention.

FIG. 6 is a graphical representation of the relative output voltages ofthe phase converter of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a phase converter embodying features of the presentinvention includes first and second input terminals IN1 and IN2, first,second and third output terminals OUT1, OUT2 and OUT3, a means forstoring electricity in the form of a first capacitor C1, a first circuitmeans for charging the first capacitor in the form of a charging circuit10 from the first and second input terminals IN1 and IN2, a secondcircuit means for supplying a third phase to the third output terminalOUT3 in the form of an output circuit 11, and a controller 12. The firstand second input terminals IN1 and IN2 are generally connected to anelectrical single phase alternating current (AC) voltage source such asa socket or two terminals connected to a utility company step-downtransformer. In the illustrated embodiment the voltage between first andsecond input terminals IN1 and IN2 may be 480 VAC. Other voltages aresuitable. The end of the capacitor C1 that is positively charged by thecharging circuit will hereinafter be designated the positive end and theopposite end will be designated the negative end. Preferably, thestorage capacitor C1 is an electrolytic capacitor having a positive anda negative end.

The charging circuit 10 has an input H bridge 14, a first inductor L1and a first current transducer CT1. The input H bridge 14 includesfirst, second, third and fourth input switches, shown as first, second,third and fourth input transistors Z1, Z2, Z3 and Z4, first, second,third and fourth input diodes D1, D2, D3, and D4 The first currenttransducer CT1 may be a current transformer, a Hall-effect currentsensor or other device. In the illustrated embodiment the first currenttransducer has a primary winding and a secondary winding. One end of thefirst inductor L1 is connected to the first input terminal IN1 and theother end of the first inductor L1 is connected to one end of theprimary winding of the first current transducer CT1.

The anode of the first input diode D1, the cathode of the second inputdiode D2, the emitter of the first input transistor Z1 and the collectorof the second input transistor Z2 all are connected to the other end ofthe primary winding of the first current transducer CT1. The cathode ofthe first input diode D1 and the collector of the first input transistorZ1 are connected to the positive end of the first capacitor C1. Theanode of the second input diode D2 and the emitter of the second inputtransistor Z2 are connected to the negative end of the first capacitorC1.

The anode of the third input diode D3, the cathode of the fourth inputdiode D4, the emitter of the third input transistor Z3 and the collectorof the fourth input transistor Z4 all are connected to the second inputterminal IN2. The cathode of the third input diode D3 and the collectorof the third input transistor Z3 are connected to the positive end ofthe first capacitor C1. The anode of the fourth input diode D4 and theemitter of the fourth input transistor Z4 are connected to the negativeend of the first capacitor C1.

The output circuit 11 has an output H bridge, a second inductor L2, asecond current transducer CT2, a transformer T1, second, third, fourthand fifth capacitors C2, C3, C4 and C5, and first, second and thirdoutput terminals OUT1, OUT2 and OUT3. The output H bridge includesfirst, second, third and fourth output switches, shown as first, second,third and fourth output transistors Z5, Z6, Z7 and Z8, and first,second, third and fourth output diodes D5, D6, D7 and D8. The secondinductor L2 and the second current transformer each include a primarywinding and a secondary winding. The cathodes of the first and thirdoutput diodes D5 and D7 and the collectors of the first and third outputtransistors Z5 and Z7 are connected to the positive end of the firstcapacitor C1. The anodes of the second and fourth output diodes D6 andD8 and the emitters of the second and fourth output transistors Z6 andZ8 are connected to the negative end of the first capacitor C1.

The anode of the first output diode D5, the cathode of the second outputdiode D6, the emitter of the first output transistor Z5 and thecollector of the second output transistor Z6 are connected to one end ofthe primary winding of the second inductor L2. The other end of theprimary winding of the second inductor L2 is connected to one end of thesecond capacitor C2 and to one end of the primary winding of thetransformer T1. The anode of the third output diode D7, the cathode ofthe fourth output diode D8, the emitter of the third output transistorZ7 and the collector of the fourth output transistor Z8 are connected toone end of the primary winding of the second current transducer CT2. Theother end of the primary winding of the second current transducer CT2 isconnected to the other end of the second capacitor C2 and the other endof the primary winding of the transformer T1.

One end of the secondary winding of the transformer T1 and one end ofthe third capacitor C3 are connected to the second output terminal OUT2and to one end of the fourth capacitor C4. The other end of the fourthcapacitor C4 is connected to the first output terminal OUT1. The otherend of the secondary winding of the transformer T1 and the other end ofthe third capacitor C3 are connected to the third output terminal T3.One end of the fifth capacitor C5 is connected to the first outputterminal OUT1 and the other end of the fifth capacitor C5 is connectedto the third output terminal OUT3. The first input terminal IN1 isconnected to the first output terminal OUT1 and the second inputterminal IN2 is connected to the second output terminal OUT2.

The controller 12 includes a plurality of connections to the abovedescribed circuit that are described by way of example, and not as alimitation, hereinafter as terminals. The first, second, third, andfourth positive transistor terminals VGE1+, VGE2+, VGE3+, and VGE4+ areconnected to the gates of the first, second, third and fourth inputtransistors Z1, Z2, Z3 and Z4. The first, second, third and fourthnegative transistor terminals VGE1−, VGE2−, VGE3− and VGE4− areconnected to the emitters of the first, second, third and fourth inputtransistors Z1, Z2, Z3 and Z4. The fifth, sixth, seventh and eighthpositive transistor terminals VGE5+, VGE6+, VGE7+, and VGE8+ areconnected to the gates of the first, second, third and fourth outputtransistors Z5, Z6, Z7 and Z8. The fifth, sixth, seventh and eighthnegative transistor terminals VGE5−, VGE6−, VGE7−, and VGE8− areconnected to the emitters of the first, second, third and fourth outputtransistors Z5, Z6, Z7 and Z8.

The voltage in terminal Vin is connected to the first input terminalIN1, the voltage out terminal Vout is connected to the third outputterminal OUT3 and the signal common terminal SC is connected to thesecond input terminal IN2. The positive and negative current interminals Iin+ and Iin− are connected to opposite ends of the secondarywinding of the first current transducer CT1. The positive and negativecurrent out terminals Iout+ and Iout− are connected to opposite ends ofthe secondary winding of the second current transducer CT1. The positiveand negative first capacitor voltage terminals VC1+ and VC1− areconnected to opposite ends of the secondary winding of the secondinductor L2. The positive and negative transformer voltage terminalsVdc+ and Vdc− are connected to opposite ends of the primary winding ofthe transformer T1.

As shown in FIG. 2, controller 12 includes a microcontroller M1, first,second, third, fourth and fifth buffer amplifiers BA1, BA2, BA3, BA4 andBA5, a first input circuit CIR1, a second input circuit CIR2, and first,second, third, fourth, fifth, sixth, seventh and eighth output driversOD1, OD2, OD3, OD4, OD5, OD6, OD7 and OD8. The input of the first bufferamplifier BA1 is connected to the voltage in terminal Vin and the outputof the first buffer amplifier BA1 is connected to the microcontrollerM1. The input of the second buffer amplifier BA2 is connected to thevoltage out terminal Vout and the output of the second buffer amplifierBA2 is connected to the microcontroller M1.

The input of the third buffer amplifier BA3 is connected to the positivecurrent in terminal Iin+ and the output of the third buffer amplifierBA3 is connected to the microcontroller M1. The input of the fourthbuffer amplifier BA4 is connected to the positive current out terminalIout+ and the output of the fourth buffer amplifier BA4 is connected tothe microcontroller M1. The input of the fifth buffer amplifier BA5 isconnected to the first input circuit CIR1 and the output of the fifthbuffer amplifier BA5 is connected to the microcontroller M1.

The first input circuit CIR1 includes a first resistor R1, first,second, third and fourth controller diodes D9, D10, D11, D12, and sixthand seventh capacitors C6 and C7. The anode of the first controllerdiode D9 and the cathode of the third controller diode D11 are connectedto the positive first capacitor voltage terminal VC1+, and the anode ofthe second controller diode D10 and the cathode of the fourth controllerdiode D12 are connected to the negative first capacitor voltage terminalVC1−. The sixth capacitor C6 is connected between the cathodes of thefirst and second controller diodes D9 and D10, and the input to thefifth buffer amplifier BA5. The anodes of the third and fourthcontroller diodes D11 and D12 are connected to ground. The seventhcapacitor C7 and the first resistor R1 each connect between the input tothe fifth buffer amplifier BA5 and ground.

The second input circuit CIR2 includes second, third and fourthresistors R2, R3 and R4, an eighth capacitor C8 and a sixth bufferamplifier BA6. One end of the second resistor R2 is connected to thepositive transformer voltage terminal Vdc+ and one end of the thirdresistor R3 is connected to the negative transformer voltage terminalVdc−. The other ends of the second and third resistors R2 and R3 areeach connected to one end of the fourth resistor R4, one end of theeighth capacitor C8 and to the negative input of the sixth bufferamplifier BA6. The positive input of the sixth buffer amplifier BA6 isconnected to Vref, and the controls of the sixth buffer amplifier BA6are connected to Vcc and ground. The other ends of the fourth resistorR4 and the eighth capacitor C8, and the output of the sixth bufferamplifier BA6 are connected together to the microcontroller M1.

The inputs of the first, second, third, fourth, fifth, sixth, seventhand eighth output drivers OD1, OD2, OD3, OD4, OD5, OD6, OD7 and OD8 areeach connected to the microcontroller M1. The positive outputs of thefirst, second, third, fourth, fifth, sixth, seventh and eighth outputdrivers OD1, OD2, OD3, OD4, OD5, OD6, OD7 and OD8 are connected to thefirst, second, third, fourth, fifth, sixth, seventh and eighth positivetransistor terminals VGE1+, VGE2+, VGE3+, VGE4+, VGE5+, VGE6+, VGE7+,and VGE8+, respectively. The negative outputs of the first, second,third, fourth, fifth, sixth, seventh and eighth output drivers OD1, OD2,OD3, OD4, OD5, OD6, OD7 and OD8 are connected to the first, second,third, fourth, fifth, sixth, seventh and eighth negative transistorterminals VGE1−, VGE2−, VGE3−, VGE4−, VGE5−, VGE6−, VGE7−, and VGE8−,respectively. The negative current in terminal Iin−, the negativecurrent out terminal Iout− and the signal common terminal SG areconnected to ground.

Referring to FIGS. 1 and 2, the first, second third and fourth inputdiodes D1, D2, D3 and D4 rectify input voltage and charge the firstcapacitor C1. During the input positive half cycle, when the voltage atthe first input terminal IN1 is greater that the voltage at the secondinput terminal, current flows through the first and fourth input diodesD1 and D4. During this input positive half cycle, the third inputtransistor Z3 is switched on and off at a high frequency with a variableduty cycle with pulse width modulation (PWM) to induce current flowthrough the first inductor L1. The PWM switching of the third inputtransistor Z3 in combination with the first inductor L1 can charge thefirst capacitor C1 above maximum voltage between the first and secondinput terminals IN1 and IN2 and the PWM duty cycles are selected toprovide a sinewave current flow through the first inductor L1, as fullydescribed in U.S. Pat. No. 6,297,971. During the input negative halfcycle, the fourth input transistor Z4 is switched with PWM.

Power can be directed from the first capacitor C1 back through the firstand second input terminals IN1 and IN2 to the voltage source by closingthe fourth input transistor Z4 and PWM switching the first inputtransistor Z1 during the input positive half cycle, and closing thethird input transistor Z3 and PWM switching the second input transistorZ2 during the input negative half cycle. The input H Bridge 14 incombination with the first inductor L1 controllably charges the firstcapacitor C1 while providing a sinewave input current and controllablydirects power from the first capacitor C1 back to the voltage source asneeded.

The output circuit 11 functions in a similar fashion. During the outputpositive half cycle, when the voltage supplied to the third outputterminal OUT3 is positive, the first output transistor Z5 is turned onand the fourth output transistor Z8 is PWM switched to provide asinusoidal positive voltage at the third output terminal OUT3. Duringthe output negative half cycle the second output transistor Z6 is closedand the third output transistor Z7 is PWM switched, reversing thedirection of current flow through the transformer T1 and providing asinusoidal negative voltage to the third output terminal OUT3. Thefirst, second, third and fourth output diodes D5, D6, D7 and D8 form afull-wave rectifier bridge so that when the voltage across the load isgreater than the voltage across the first capacitor C1, power will befed back to the first capacitor C1.

The controller 12 monitors the input and output voltages and currents,and switches the first, second, third and fourth input and outputtransistors Z1, Z2, Z3, Z4, Z5, Z6, Z7 and Z8 appropriately. The inputvoltage is sampled at the voltage in terminal Vin, and the generatedphase is sampled at the voltage out terminal Vout. The signal commonterminal SC is used for the reference point. The current through thefirst inductor L1 is measured using the first current transducer CT1 andthe second current transducer CT2 measures the current through thesecond inductor L2.

The peak-to-peak value of the high frequency signal across the secondinductor L2 is exactly equal to the DC value of the voltage across thefirst capacitor C1. The voltage across the first capacitor C1 ismeasured by applying the signal across the first capacitor voltageterminals VC1+ and VC1−, which are connected to opposite ends of thesecondary winding of the second inductor L2, to the full-wave rectifierof the first input circuit CIR1. The high-pass filter, in the firstinput circuit CIR1, at the rectifier output gives a DC voltage directlyproportional to the first capacitor C1 voltage which can then bereferenced to the signal common terminal SC and measured by themicroprocessor M1. The same result could be obtained by using asecondary coil on the first inductor L1. The second input circuit CIR2provides the integral of the voltage across the primary winding of thetransformer T1, to assure that the transformer T1 voltage has an averagevalue of zero.

Typically the controller 12 would produce a PWM frequency between 1 kHzand 100 kHz. The controller 12 also senses for overload conditions inthe input and output current and in the individual transistors andadjusts the transistor modulation to limit the transistor current tosafe values.

FIG. 3 shows a phase converter embodying features of the presentinvention, with an alternative charging circuit 10 having the first,second, third and fourth input diodes D1, D2, D3 and D4, the firstcurrent transducer CT1, the first inductor L1, the first inputtransistor Z1, and a fifth input diode D13. The anode of the first inputdiode D1 and the cathode of the second input diode D2 are connected tothe first input terminal IN1. The anode of the third input diode D3 andthe cathode of the fourth input diode D4 are connected to the secondinput terminal IN2.

The cathodes of the first and third input diodes D1 and D3 are connectedto one end of the first current transducer CT1. The other end of thefirst current transducer CT1 is connected to one end of the firstinductor L1. The other end of the first inductor L1 is connected to thecollector of the first input transistor Z1 and to the anode of the fifthinput diode D13. The cathode of the fifth input diode D13 is connectedto the positive end of the first capacitor C1. The anodes of the secondand fourth input diodes D2 and D4 are connected to the emitter of thefirst input transistor Z1 and to the negative end of the first capacitorC1.

The controller 12 includes a plurality of connections to the abovedescribed circuit that are described for the circuit in FIG. 3 as in thecircuit of FIG. 1 as terminals. The first positive transistor terminalsVGE1+ is connected to the gate of the first input transistor Z1 and thefirst negative transistor terminals VGE1− is connected to the emitter ofthe first input transistor Z1. The fifth, sixth, seventh and eighthpositive transistor terminals VGE5+, VGE6+, VGE7+, and VGE8+ areconnected to the gates of the first, second, third and fourth outputtransistors Z5, Z6, Z7 and Z8. The fifth, sixth, seventh and eighthnegative transistor terminals VGE5−, VGE6−, VGE7−, and VGE8− areconnected to the emitters of the first, second, third and fourth outputtransistors Z5, Z6, Z7 and Z8.

The voltage in terminal Vin is connected to the first input terminalIN1, the voltage out terminal Vout is connected to the third outputterminal OUT3 and the signal common terminal SC is connected to thesecond input terminal IN2. The positive and negative current interminals Iin+ and Iin− are connected to opposite ends of the secondarywinding of the first current transducer CT1. The positive and negativecurrent out terminals Iout+ and Iout− are connected to opposite ends ofthe secondary winding of the second current transducer CT1. The positiveand negative first capacitor voltage terminals VC1+ and VC1− areconnected to opposite ends of the secondary winding of the secondinductor L2. The positive and negative transformer voltage terminalsVdc+ and Vdc− are connected to opposite ends of the primary winding ofthe transformer T1.

Referring to FIG. 4, controller 12 includes a microcontroller M1, first,second, third, fourth and fifth buffer amplifiers BA1, BA2, BA3, BA4 andBAS, a first input circuit CIR1, a second input circuit CIR2, and first,fifth, sixth, seventh and eighth output drivers OD1, OD5, OD6, OD7 andOD8. The input of the first buffer amplifier BA1 is connected to thevoltage in terminal Vin and the output of the first buffer amplifier BA1is connected to the microcontroller M1. The input of the second bufferamplifier BA2 is connected to the voltage out terminal Vout and theoutput of the second buffer amplifier BA2 is connected to themicrocontroller M1.

The input of the third buffer amplifier BA3 is connected to the positivecurrent in terminal Iin+ and the output of the third buffer amplifierBA3 is connected to the microcontroller M1. The input of the fourthbuffer amplifier BA4 is connected to the positive current out terminalIout+ and the output of the fourth buffer amplifier BA4 is connected tothe microcontroller M1. The input of the fifth buffer amplifier BA5 isconnected to the first input circuit CIR1 and the output of the fifthbuffer amplifier BAS is connected to the microcontroller M1.

The first input circuit CIR1 includes a first resistor R1, first,second, third and fourth controller diodes D9, D10, D11, D12, and sixthand seventh capacitors C6 and C7. The anode of the first controllerdiode D9 and the cathode of the third controller diode D11 are connectedto the positive first capacitor voltage terminal VC1+, and the anode ofthe second controller diode D10 and the cathode of the fourth controllerdiode D12 are connected to the negative first capacitor voltage terminalVC1−. The sixth capacitor C6 is connected between the cathodes of thefirst and second controller diodes D9 and D10, and the input to thefifth buffer amplifier BA5. The anodes of the third and fourthcontroller diodes D11 and D12 are connected to ground. The seventhcapacitor C7 and the first resistor R1 each connect between the input tothe fifth buffer amplifier BA5 and ground.

The second input circuit CIR2 includes second, third and fourthresistors R2, R3 and R4, an eighth capacitor C8 and a sixth bufferamplifier BA6. One end of the second resistor R2 is connected to thepositive transformer voltage terminal Vdc+ and one end of the thirdresistor R3 is connected to the negative transformer voltage terminalVdc−. The other ends of the second and third resistors R2 and R3 areeach connected to one end of the fourth resistor R4, one end of theeighth capacitor C8 and to the negative input of the sixth bufferamplifier BA6. The positive input of the sixth buffer amplifier BA6 isconnected to Vref, and the controls of the sixth buffer amplifier BA6are connected to Vcc and ground. The other ends of the fourth resistorR4 and the eighth capacitor C8, and the output of the sixth bufferamplifier BA6 are connected together to the microcontroller M1.

The inputs of the first, fifth, sixth, seventh and eighth output driversOD1, OD5, OD6, OD7 and OD8 are each connected to the microcontroller M1.The positive outputs of the first, fifth, sixth, seventh and eighthoutput drivers OD1, OD5, OD6, OD7 and OD8 are connected to the first,fifth, sixth, seventh and eighth positive transistor terminals VGE1+,VGE5+, VGE6+, VGE7+, and VGE8+, respectively. The negative outputs ofthe first, fifth, sixth, seventh and eighth output drivers OD1, OD5,OD6, OD7 and OD8 are connected to the first, fifth, sixth, seventh andeighth negative transistor terminals VGE1−, VGE5−, VGE6−, VGE7−, andVGE8−, respectively. The negative current in terminal Iin−, the negativecurrent out terminal Iout− and the signal common terminal SG areconnected to ground.

The phase converter shown in FIGS. 3 and 4 functions in a similarfashion the phase converter shown in FIGS. 1 and 2, except that powercannot be fed back from the first capacitor C1 to the first and secondinput terminals IN1 and IN2. The first, second, third and fourth inputdiodes D1, D2, D3 and D4 form a full-wave rectifier. The first inputtransistor Z1 is PWM switched to, in combination with the first inductorL1, provide draw a sinusoidal input current from the first and secondinput terminals IN1 and IN2. The fifth input diode D13 prevents reversecurrent flow between the positive and negative ends of the firstcapacitor C1.

FIG. 5 shows the voltages V₁, V₂ and V₃ at the first, second and thirdoutput terminals OUT1, OUT2 and OUT3, respectively, over a period ofabout 30 milliseconds for an input source of 240 VAC across the firstand second input terminals IN1 and IN2. The voltages V₁ and V₂ are about120 Vrms with a maximum amplitude of 120×{square root over (2)}=170V andare 180 degrees out of phase. The maximum amplitude of voltage V₃ is170×{square root over (3)}=294V and voltage V₃ is 90 degrees out phasewith V₁ and V₂. FIG. 6 shows these same voltages as the voltages V₃₂,V₁₃ and V₂₁ between the first, second and third output terminals OUT1,OUT2 and OUT3. In this way, by increasing the amplitude of the voltageat the third output terminal OUT3 and shifting the phase by 90 degrees,balanced three phase AC power is produced by the converter and suppliedto the load.

The static phase converter of the present invention, unlike many priorknown devices, does not require that the components be selected to matcha specific load and provides balanced three phase AC power over a rangeof loads. This static phase converter is more efficient, less complexand less expensive than prior known inverters. This phase converterprovides sinusoidal input current instead of highly peaked inputcurrent, preventing negative effects on the power grid. This phaseconverter can be used to supply power to inductive, capacitive orresistive loads.

The phase converter of the present invention does not require thevoltage doubling required by the phase converter of U.S. Pat. No.6,297,971, because the output H bridge 15 switches the output of thefirst capacitor C1. In the illustrated embodiment the output H bridge 15switches the output of the first capacitor C1 by cyclically reversingthe current direction through the primary winding of the transformer T1.The bus capacitor voltage in the present invention is half that of theprior version, and therefore switches with half the voltage rating canbe used. Higher input voltages are possible with the same switches usedin the prior version, or faster and less expensive switches can be usedfor the prior input voltage.

Although the present invention has been described with a certain degreeof particularity, it is understood that the present disclosure has beenmade by way of example and that changes in details of structure may bemade without departing from the spirit thereof.

What is claimed is:
 1. A phase converter for converting single phase AC power to balanced three phase AC power comprising: first and second input terminals, first, second and third output terminals for connection to a three phase load, said first output terminal being directly connected to said first input terminal and said second output terminal being directly connected to said second input terminal, a capacitor having a positive end and a negative end, first circuit means, connected to said first and second input terminals, to said positive end of said capacitor and to said negative end of said capacitor, for controllably charging said capacitor with a voltage, and second circuit means, connected to said positive end and to said negative end of said capacitor, and coupled to said third output terminal, for controllably discharging said capacitor while cyclically reversing voltage to said third output terminal between positive and negative, for supplying single phase AC power of a predetermined amplitude and phase, relative to the amplitude and phase of the power at said first and second output terminals, to said third output terminal, to result in balanced three phase AC power to said first, second and third output terminals.
 2. The converter as set forth in claim 1 wherein said second circuit means includes an output H bridge, connected to said positive and negative ends of said capacitor, that cyclically couples said positive end of said capacitor to said third terminal and then said negative end of said capacitor to said third terminal.
 3. The converter as set forth in claim 2 wherein said second circuit means includes an inductor and a transformer having a primary winding and a secondary winding, said inductor having a first end of connected to said output H bridge and a second end, said primary winding having a first end connected to said second end of said inductor and a second end connected to said output H bridge, and said secondary winding having a first end connected to said third terminal.
 4. The converter as set forth in claim 3 wherein said output H bridge includes first, second, third and fourth output diodes each having an anode and a cathode and first, second, third and fourth output switches each having an input, an output and a control terminal, with said inputs of said first and third output switches and said cathodes of said first and third output diodes being connected to said positive end of said capacitor, said outputs of said second and fourth output switches and said anodes of said second and fourth output diodes being connected to said negative end of said capacitor, said output of said first output switch, said input of said second output switch, said anode of said first output diode and said cathode of said second output diode being connected to said first end of said inductor, and said output of said third output switch, said input of said fourth output switch, said anode of said third output diode and said cathode of said fourth output diode being connected to said second end of said primary winding of said transformer.
 5. The converter as set forth in claim 4 including a controller, connected to said control terminals of said first, second, third and fourth output switches, that selectively pulse width modulation switches said first, second, third and fourth output switches.
 6. The converter as set forth in claim 1 wherein said first circuit means includes an inductor having a first end connected to said first input terminal and a second end, and an input H bridge connected to said second end of said inductor, said second input terminal and said positive and negative ends of said capacitor.
 7. The converter as set forth in claim 6 wherein said input H bridge includes first, second, third and fourth input diodes each having an anode and a cathode and first, second, third and fourth input switches each having an input, an output and a control terminal, with said output of said first input switch, said input of said second input switch, said anode of said first input diode and said cathode of said second diode being connected to said second end of said inductor, said output of said third input switch, said input of said fourth input switch, said anode of said third input diode and said cathode of said fourth input diode being connected to said second input terminal, said inputs of said first and third input switches and said cathodes of said first and third input diodes being connected to said positive end of said capacitor, and said outputs of said second and fourth input switches and said anodes of said second and fourth input diodes being connected to said negative end of said capacitor.
 8. The converter as set forth in claim 7 including a controller, connected to said control terminals of said first, second, third and fourth input switches, that selectively pulse width modulation switches said first, second, third and fourth input switches.
 9. The converter as set forth in claim 1 wherein said first circuit means includes an inductor having a first end and a second end, first, second, third, fourth and fifth input diodes each having an anode and a cathode and an input switch having an input, an output and a control terminal, with anode of said first input diode and said cathode of said third input diode being connected to said first input terminal, said anode of said second input diode and said cathode of said fourth input diode being connected to said second input terminal, said cathodes of said first and second input diodes being connected to said first end of said inductor, said input of said input switch and said second end of said inductor being connected to said anode of said fifth input diode, said cathode of said fifth input diode being connected to said positive end of said capacitor, and said anodes of said third and fourth input diodes and said output of said switch being connected to said negative end of said capacitor.
 10. The converter as set forth in claim 9 including a controller, connected to said control terminal said input switch, that selectively pulse width modulation switches said input switch.
 11. A phase converter for converting single phase AC power to balanced three phase AC power comprising: first and second input terminals, first, second and third output terminals for connection to a three phase load, said first output terminal being directly connected to said first input terminal and said second output terminal being directly connected to said second input terminal, a capacitor having a positive end and a negative end, a first inductor having a first end, connected to said first input terminal, and a second end, an input H bridge including first, second, third and fourth input diodes each having an anode and a cathode, and first, second, third and fourth input switches each having an input, an output and a control terminal, with said output of said first input switch, said input of said second input switch, said anode of said first input diode and said cathode of said second diode being connected to said second end of said first inductor, said output of said third input switch, said input of said fourth input switch, said anode of said third input diode and said cathode of said fourth input diode being connected to said second input terminal, said inputs of said first and third input switches and said cathodes of said first and third input diodes being connected to said positive end of said capacitor, and said outputs of said second and fourth input switches and said anodes of said second and fourth input diodes being connected to said negative end of said capacitor a second inductor having a first end and a second end, a transformer having a primary winding, and a secondary winding, said primary winding having a first end connected to said first end of said second inductor and a second end, said secondary winding having a first end connected to said third output terminal and a second end, an output H bridge including first, second, third and fourth output diodes each having an anode and a cathode, and first, second, third and fourth output switches each having an input, an output and a control terminal, with said inputs of said first and third output switches and said cathodes of said first and third output diodes being connected to said positive end of said capacitor, said outputs of said second and fourth output switches and said anodes of said second and fourth output diodes being connected to said negative end of said capacitor, said output of said first output switch, said input of said second output switch, said anode of said first output diode and said cathode of said second output diode being connected to said first end of said second inductor, and said output of said third output switch, said input of said fourth output switch, said anode of said third output diode and said cathode of said fourth output diode being connected to said second end of said primary winding of said transformer, and a controller, connected to said control terminals of said first, second, third and fourth input switches and said first, second, third and fourth output switches, that selectively pulse width modulation switches said first, second, third and fourth input switches and said first, second, third and fourth output switches.
 12. A phase converter for converting single phase AC power to balanced three phase AC power comprising: first and second input terminals, first, second and third output terminals for connection to a three phase load, said first output terminal being directly connected to said first input terminal and said second output terminal being directly connected to said second input terminal, a capacitor having a positive end and a negative end, first, second, third and fourth input diodes each having an anode and a cathode, with said anode of said first input diode and said cathode of said third input diode being connected to said first input terminal, said anode of said second input diode and said cathode of said fourth input diode being connected to said second input terminal, and said anodes of said third and fourth input diodes being connected to said negative end of said capacitor, a first inductor having a first end and a second end, with said first end of said first inductor being connected to said cathodes of said first and second input diodes, an input switch having an input, an output and a control terminal, with said output of said switch being connected to said negative end of said capacitor a fifth input diode having an anode and a cathode, said anode of said fifth input diode being connected to said input of said input switch and said second end of said inductor, said cathode of said fifth input diode being connected to said positive end of said capacitor, a second inductor having a first end and a second end, a transformer having a primary winding, and a secondary winding, said primary winding having a first end connected to said first end of said second inductor and a second end, said secondary winding having a first end connected to said third output terminal and a second end, an output H bridge including first, second, third and fourth output diodes each having an anode and a cathode, and first, second, third and fourth output switches each having an input, an output and a control terminal, with said inputs of said first and third output switches and said cathodes of said first and third output diodes being connected to said positive end of said capacitor, said outputs of said second and fourth output switches and said anodes of said second and fourth output diodes being connected to said negative end of said capacitor, said output of said first output switch, said input of said second output switch, said anode of said first output diode and said cathode of said second output diode being connected to said first end of said second inductor, and said output of said third output switch, said input of said fourth output switch, said anode of said third output diode and said cathode of said fourth output diode being connected to said second end of said primary winding of said transformer, and a controller, connected to said control terminals of said input switch and said first, second, third and fourth output switches, that selectively pulse width modulation switches said input switch and said first, second, third and fourth output switches. 